The present invention relates to an input device utilizing a transistor for a tuner.
In the input device for a tuner utilizing a tetrode metal-oxide-semiconductor field-effect transistor (to be referred to as "tetrode MOSFET" hereinafter for brevity), the input impedance of the MOSFET may be expressed in terms of a resistor and a capacitor connected in parallel. When the frequency increases, the input capacitance increases only a little whereas the input resistance decreases very remarkably so that the tuning frequency band widths or selectivities are widely different in the high and low frequency bands when the impedance matching between an antenna and the MOSFET is established. Especially in the low frequency band, the tuning band width or selectivity becomes very narrow or very sharp so that the special adjustment is required. This is not preferable from the standpoint of assembly and electrical characteristics.